Commercial jet aircraft currently are required to meet governmentally specified perceived noise criteria. The prior systems used in engine silencing include both passive and active methods and apparatus. Passive means include mufflers, acoustic treatment in the inlets and exhaust ducts and tailpipes. Active means include engine surge control, turbofans with high bypass ratios, bypass/exhaust mixers, external suppressors, and acoustic wave interference in engine inlet ducts, exhaust tailpipes, and mufflers.
The previous art, for example, Wanke U.S. Pat. No. 3,936,606, issued Feb. 3, 1976, utilized acoustic wave interference to achieve sound reduction in gas turbine engines. A microphone, or equivalent, measured the existing acoustic wave. Conventional adaptive control apparatus created a time-delayed and phase reversed mirror symmetry signal to generate an anti-noise acoustic wave downstream from the microphone via a "speaker" in a turbojet's inlet duct and exhaust tailpipe. The Wanke patent, however required detailed modelling of the acoustic wave and the counter wave that would cancel it. Since the wave that effected cancellation was a simple time delayed and phase reversed wave front, Wanke found it necessary to direct the waves through a wave guide that converted all the wave energy of both the acoustic wave and the counter wave into plane waves or other predictable wave modes. Cancellation could only be accomplished within such wave guides, and therefore they had to be present within the region where the acoustic energy would be canceled. The use of such conventional noise control systems has had limited results in turbojet engine inlet ducts.
Other prior systems of active noise suppression on gas turbines has not produced cancellation of non-linear, random noise over the needed acoustic frequency range in real time.
The prior methods and apparatus for active sound control, noise cancellation, noise abatement, noise attenuation, and the like, involve conventional adaptive controllers or adaptive filters. These systems require extensive system modeling in order to operate successfully. They have limited abilities in non-stationary and non-linear acoustic applications.